The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in formations containing oil shale. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbonaceous product is called "shale oil". Liquid products from an in situ oil shale retort can also include water which appears as a separate phase or as a complex emulsion of crude shale oil and water. "Crude shale oil" is a term used herein for shale oil withdrawn from a fragmented permeable mass of formation particles in an in situ oil shale retort without further processing, except such processing as may be required for separating crude shale oil from water. Such processing can include heating of the crude shale and water, such as an emulsion of crude shale oil and water, to about 150.degree. F. or more for about a day, addition of minor amounts of emulsion breaking materials, or both.
A number of methods have been proposed for processing the oil shale which involve either mining the kerogen-bearing shale and processing the oil shale above ground or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated oil shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. Such patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-containing retort inlet mixture into the retort as an oxygen-supplying gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the retort, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products, including gaseous and liquid hydrocarbonaceous products, and to a residual solid carbonaceous material. Residual carbonaceous material in the retorted oil shale provides fuel for advancing the combustion zone through the retorted oil shale.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbonaceous products together with water produced in or added to the retort, are collected at the bottom of the retort. The liquid hydrocarbonaceous products are separated from the water in the liquid product stream as crude shale oil. An off gas containing combustion gas, including carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process, is also withdrawn from the bottom of the retort.
Kerogen contains appreciable amounts of sulfur, and such sulfur can appear in hydrocarbonaceous materials such as crude shale oil. Sulfur can also be present in an in situ retort in mineral sulfides such as pyrite. Some sulfur is withdrawn from the in situ retort as hydrogen sulfide or other sulfurous gases in the off gas. An appreciable amount of the sulfur, however, can be present in the crude shale oil from an in situ retort, often exceeding about one percent by weight. It is desirable to remove sulfur from crude shale oil since it is a contaminant which can have adverse effects in subsequent use or processing of the crude shale oil. For example, if the crude shale oil is used directly as a burner feed, sulfur can appear in the combustion gas and must be scrubbed before released to the environment. It is also desirable to have a low sulfur crude oil for refinery or petrochemical feed stocks.
Many processes have been described for removing sulfur from liquid hydrocarbonaceous fuels. Many such processes involve selective oxidation of sulfur compounds with oxygen or nitrogen oxides. After oxidation, a thermal decomposition step may be practiced at temperatures above about 200.degree. C. to insure that substantially all of the gaseous decomposition products are given off. Sulfur is liberated mainly as sulfur dioxide, although at higher temperatures in the region of 300.degree. C. and over increasing quantities of hydrogen sulfide are also liberated. Such a process is described, for example, in U.S. Pat. No. 3,341,448 by Ford et al. Such treatment is applied to oil treated by oxidation rather than untreated crude shale oil.
Other processes involve catalytic hydro-desulfurization in the presence of elemental hydrogen to produce volatile hydrogen sulfide, which is removed from the liquid fuel. U.S. Pat. No. 3,106,521 by Huntington describes a method for producing shale oil having a low sulfur content which involves retorting oil shale in an atmosphere of hydrogen at high temperatures and pressures of about 15 to 30 atmospheres and catalytically hydrofining volatile products of retorting at 15 to 30 atmospheres pressure. In the hydrofining step, sulfur is converted to gaseous hydrogen sulfide which is removed when the hydrofining product is condensed to the liquid state.
According to U.S. Pat. No. 3,919,402, by Guth et al, which relates to desulfurization of petroleum, certain petroleums such as Beaumont Oil can be desulfurized by passage through a still at temperatures on the order of 300.degree. to 500.degree. F. to remove sulfur as hydrogen sulfide. This is stated to be a special characteristic of Beaumont Oil; not all petroleums give off hydrogen sulfide upon heating.
In Petroleum Refining Engineering by W. Nelson, Second Edition, McGraw Hill Book Company, Inc., New York, 1941, page 606, it is stated that most sulfur compounds are decomposed by heating to 700.degree. to 750.degree. F. in the presence of bauxite, and a process is described in which sulfur is removed from gasoline vapor at about 645.degree. F. and a pressure of 50 pounds per square inch in the presence of a hydrated silicate of alumina catalyst which contains oxides of nickel, cobalt, and other metals. However, the catalyst is subject to poisoning in a period of three to ten hours and must periodically be regenerated.
U.S. Pat. No. 3,284,336 by Culbertson et al describes a method for reducing pour point of shale oil by fractionating the oil to obtain a light fraction and an oil residue, heat treating the oil residue at a temperature above about 600.degree. F. and below the point of incipient thermal decomposition of the residue. The heat treated residue is recombined with the light fraction to result in an oil having a pour point lower than that of the original crude oil. The desired modification of the heavy fraction is effected without formation of appreciable amounts of non-condensible gases. Temperatures are preferably 600.degree. F. (315.degree. C.) to 800.degree. F. (427.degree. C.). No mention is made of treating the crude shale oil for reducing sulfur content.
Other references concerning sulfur in petroleum include U.S. Pat. Nos. 1,935,207 by Harder et al; 1,968,842 by Malisoff; 2,489,316 by Proell; 2,489,318 by Proell; 2,505,910 by Proell et al; 2,573,674 by Adams et al; 2,581,050 by Smedslund; 2,598,013 by Proell et al; 2,598,014 by Proell et al; 2,702,824 by Wetterholm; 2,825,744 by Smedslund; 2,825,745 by Smedslund; 3,135,680 by Fierce et al; 3,164,546 by Millikan et al; 3,244,618 by Diamond et al; 3,267,027 by Fierce et al, 3,294,677 by Martin et al; 3,847,800 by Guth et al; 3,953,180 by Hoffert et al; and 3,975,303 by Eyles et al.
The need for low sulfur fuels is great and increasing. Moreover, the importance of shale oil as fuel is expected to increase steadily as the cost of pretroleum rises and the supply dwindles. Consequently, reliable, low cost methods of removing sulfur from shale oil are needed.